Field of Invention
The present invention relates to a method for producing zinc oxide, and more particularly to a method for producing high-purity nanometer zinc oxide.
Description of Related Arts
At present, the smoke and dust from steel plant (including blast furnace grey, converter ash, electric furnace ash), also called smoke and dust storage ash, each producing a ton of steel will produce 35˜90 kg of said smoke and dust, said smoke and dust contains iron of 15˜30%, silicon oxide of 4˜5%, zinc of 5˜22%, combustion fixed carbon (C) of 25˜55%, calcium oxide of 2˜5%, magnesium oxide of 1˜2%, and titanium, vanadium and alkali etc. Under normal conditions, said smoke and dust is generally used as sintering raw materials to produce sinter production, it is recycled in steel plant internal, with enrichment of the cycle, the zinc load entering into furnace is more and more high, seriously affect the normal operation of Blast furnace.
Methods currently limit zinc load in blast furnace contain: one is limiting the amount of recycled smoke and dust; two is mineral processing by the smoke and dust; three is adopting pyrometallurgy and hydrometallurgy. The first method is not an economic and effective method for reducing zinc load of blast furnace, and it brings environmental pollution. The second method is enriching the zinc to the tail mud, but in the mud, but the three products of fine iron, fine carbon and tail mud is disorder, iron, carbon resource are still lost. The third method contains pyrometallurgy and hydrometallurgy, the pyrometallurgy is divided into the treatment of direct sintering method, pelletizing treatment method and direct reduction processing method. But zinc, lead and alkali metals have not been solved. The hydrometallurgy is divided into acid method and alkali method, the process of acid method is maturity, the zinc leaching rate is only about 80% if it is no heating, if rising the temperature, the zinc leaching rate is up to 95%, but the iron is as high as 60%, iron removal is difficult, and a waste of iron, serious corrosion of equipment, is not up to the requirements of environmental protection. But the leaching rate of alkali method is lower. The overall characteristic of existing hydrometallurgy leaching zinc is the zinc leaching rate is low, leached residue is difficult to recycle, unable to meet the requirements of environmental protection, serious equipment corrosion, sensitive to the material requirements, difficult optimization for process , production efficiency is low and the steel output does not match. At present, China's iron and steel enterprises dust containing zinc adding sintering recycling mode has on blast furnace, sintering production and steel plant environment brought great harm, treatment of dust is very urgent.
The most ideal processing method for the steel plant smoke and dust is the selective leaching of zinc, it make the zinc entering into the final leached solution, and recycle zinc valuably.
High-purity zinc oxide usually refers to the zinc oxide product with the mass percent ≧99.7%. The high-purity zinc oxide is an indispensable raw material for the modern high technologies, with wide applications. It is mainly used in glass, feed, ceramics, dyes, paint, paper-making, rubber, pesticides, oil refining, galvanization, special steel, alloy, defence-related science and technology, etc. The glass, paper-making, or rubber, oil refining enterprises have high demands for zinc oxide and very high purity requirement.
The current method for producing high-purity zinc oxide, mainly is the indirect method, said indirect method in general adopts zinc ingots as raw material, through the electrolytic reduction, or high temperature air gasification, oxidation and condensation collection to prepare zinc oxide, adopting different raw materials of zinc ingots, the purity of produced zinc oxide is not the same, this process is mainly the production of zinc oxide of 99.5%-99.7%.
Ammonia method is a commonly used method for producing zinc oxide. Currently, the ammonia method (ammonia—ammonium bicarbonate combined leaching method) for producing zinc oxide generally includes the following steps: leaching of zinc-containing materials using ammonia-ammonium bicarbonate as leaching agent, and after purification, ammonia evaporation crystallization, drying, calcinations of zinc-ammine complexing solution, produce the zinc oxide product. In general, the content of zinc oxide is 95-98%.
Above-mentioned traditional ammonia method for producing zinc oxide has not been used in steel plant smoke and dust, mainly due to the following reasons:                1. Those steel plant smoke and dust have low zinc content (in general, Zn %=5−22), the leached solution has low zinc concentration, high consumption of leaching agent and high cost, so enterprises can not afford;        2. Because of complicated impurities, it only can obtain product of general activity zinc oxide, qualified rate of products is low, product prices are low and economic benefit is difference;        3. Conventional means of leaching, leached rate of zinc ore is low, residual zinc in leached residue is high, resource recycle of Iron, carbon has not formed a complete chain, smoke and dust value not use and reflect.        
Nanometer zinc oxide (ZnO) is a new type of high-function fine inorganic product with the particle size between 1 and 100 nm in the 21st century, exhibiting a variety of special properties such as non-migratory, fluorescence, piezoelectric, absorption and UV scattering ability, etc. With its special optical, electrical, magnetic and sensitivity performance, it can be used to produce gas sensors, phosphors, rheostat, UV shielding materials, image recording materials, piezoelectric materials, varistors, efficient catalysts, magnetic materials, and plastic films, etc.
Currently the methods producing zinc oxide mainly include chemical precipitation method, sol-gel method, microemulsion method and hydrothermal synthesis method, etc. But all raw materials used are zinc calcine or pure zinc salts (such as zinc sulfate, zinc nitrate or zinc acetate) with the zinc content higher than 50%.
Currently, the disclosed technologies of producing nanometer zinc oxide by ammonia leaching method are low-temperature hydrolysis methods, for example:                Chinese Patent Application No. 92103230.7 discloses an improved technology for producing zinc oxide aiming at traditional ammonia complexometry. The purified zinc-ammonia complexing solution is diluted with water to hydrolyze part of zinc-ammonia complexing solution and obtain the basic zinc carbonate (with a ratio of zinc hydroxide and zinc carbonate of 2:1), and then continue to heat until zinc-ammonia complexing solution is decomposed completely. After high-temperature calcinations, 30-100 nn nanometer zinc oxide is obtained.        
For the patented technology, the following problems need to be solved:                After hydrolysis, in the thermal decomposition process of undissociated zinc-ammonia complexing solution, the newly generated basic zinc carbonate will continue to grow on the original surface of crystal nucleus, promoting the growing of originally hydrolyzed crystal, which is prone to cause uneven zinc carbonate crystal, making it difficult to control the particle size of the final product.        
It adds 4-10 times of water, reducing the efficiency in the preparation process, increasing the energy consumption and the water treatment cost at the back end.
Chinese Patent Application No. 200610130477.7 disclosed an improved technology producing zinc oxide for the traditional ammonia complex method. The zinc-ammonia complexing solution is mixed with 1:2-20 of hot water or hot mother liquor continuously. After heating and heat preservation, it is recycled to be used in hydrolysis of zinc-ammonia complexing solution, to prepare 10-50 nn of nanometer zinc oxide.
For the patented technology, the following problems need to be solved:                After hydrolysis of mother liquor, the ammonia cannot be fully separated, and it cannot achieve the hydrolysis effect, and finally the zinc-ammonia complexing solution is mixed with the zinc-ammonia complexing solution.        
For the above two patents, the nanometer crystals are obtained by slightly changing pH value with a large amount of water. In fact, relying solely on the pH value slight change, only a very small part of hydrolysis can be achieved (checked from the ammonium hydroxide solubility curve of zinc oxide). The higher concentration of zinc ammonia liquid, the higher the precipitation efficiency and lower energy consumption; while the lower concentration of zinc ammonia liquid, the lower the precipitation efficiency and high energy consumption. It is technically feasible by artificially increasing the proportion of water to produce nanometer zinc oxide, but it is not feasible in terms of economic efficiency.
In addition, for the current ammonia leaching method for producing zinc oxide, the crystal is basic zinc carbonate, with high decomposition temperature (the initial temperature of zinc hydroxide decomposition is about 125° C., and that of zinc carbonate is about 300° C.). In order to obtain high-purity products, it is necessary to guarantee a high decomposition temperature, generally controlled at above 500° C., to completely decompose the basic zinc carbonate. For example, in the Chinese Patent with Application No. 200610130477.7, the calcinations temperature is as high as 550° C. High-temperature calcinations seriously affect the specific surface area and dispersity of zinc oxide, and thereby affecting its application field.
In summary, for treatment process of the steel plant smoke and dust, how to effectively leach the zinc from materials with low zinc content and get high-purity nanometer zinc oxide and to overcome the disadvantages of traditional pyrometallurgy and hydrometallurgy have become technical problems urgently to be resolved in the industry.